KROHNE SU 501 EX EN - Manual (Page 27)

If the ratio of the internal diagnostics test rate of the measuring
system to the demand rate exceeds the value 100, the
measuring system can be treated in the way it is executing a
safety function in the mode with low demand rate (IEC 61508-2,
7.4.3.2.5).
Corresponding characteristics is the value PFD
avg
(average
Probability of dangerous Failure on Demand). It is dependent
on the test interval T
Proof
between the function tests of the
protective function.
Numbers see paragraph "Safety-technical characteristics".
If the "low demand mode" does not apply, the measuring
system must be used as safety-relevant subsystem in "high
demand" (IEC 61508-4, 3.5.12).
The fault tolerance time of the complete system must be higher
than the sum of the reaction times or the diagnostics test
periods of all components in the safety chain.
Corresponding characteristics is the value PFH (failure rate).
Numbers see paragraph "Safety-technical characteristics".
The safe condition of the measuring system is the switched off
status (quiescent current principle):
l
Relay output – relay deenergised
l
Transistor output – transistor blocks
A fail-safe failure (safe failure) exists if the measuring system
changes to the defined safe condition without demand of the
process.
If the signal conditioning instrument detects currents <3.6 mA
or >21.6 mA, the signal conditioning instrument takes on the
safe condition.
A dangerous undetected failure exists if the measuring system
does not go to the defined safe condition when required by the
process.
The processing unit must evaluate the output circuit of the
measuring system by taking the quiescent current principle
into account.
The processing unit must correspond to the SIL level of the
measuring chain.
High demand mode
Safe condition and fault
description
Configuration of the proces-
sing unit
SU 501 Ex - Signal conditioning instrument 27
Functional safety
27953
-EN
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